(1) Field of the Invention
This invention relates to an optical amplifier and, more particularly, to an optical amplifier for amplifying a wavelength division multiplexing (WDM) optical signal.
(2) Description of the Related Art
In recent years WDM communication in which a plurality of optical signals with different wavelengths are multiplexed into a single optical fiber has occupied attention and been put to practical use. One of the important devices used in WDM systems is the erbium-doped fiber amplifier (EDFA).
The EDFA is an optical amplifier in which an erbium-doped fiber (EDF) is used as an amplification medium. With the EDFA, excitation light emitted from an excitation light source (semiconductor laser) is inputted to the fiber through which an optical signal is traveling, and the level of the optical signal is amplified by a stimulated emission which occurs at that time.
By the way, the excitation light source will emit desired excitation light when a drive signal is provided thereto. However, the characteristics of the excitation light source will change due to, for example, degradation with the passage of time, so the correlation between a drive signal and excitation light power will change. For example, though a drive signal d1 was provided for making the excitation light source output excitation light power p1, a drive signal d2 (d1<d2) is required for obtaining the excitation light power p1 because of a change in the characteristics of the excitation light source caused by degradation with the passage of time. Therefore, to control the driving of the excitation light source, a drive signal according to a change in the characteristics of the excitation light source must always be provided.
Conventionally, to control the driving of an excitation light source, a technique in which compensation for the temperature and degradation of the excitation light source is made by comparing excitation light at operation time with excitation light, which is emitted when the excitation light source is driven on the basis of data for making it output desired excitation light at ambient temperature and which is treated as a reference value, has been proposed (see, for example, Japanese Unexamined Patent Publication No. 2002-217836, paragraph nos. [0017]–[0033] and FIG. 1).
With the above conventional technique, however, the degree of actual degradation of the excitation light source is not recognized and a change in input optical signals is not taken into consideration at all. Accordingly, the accuracy of the degradation compensation is low and system environments to which the conventional technique is applicable are limited.
FIG. 11 is a block diagram showing the rough structure of a conventional apparatus. A conventional apparatus 100 comprises a temperature sensor 101, data storage sections 102 and 105, an excitation control section 103, a light receiving element 104, a differential amplifier 106, and a CPU 107.
The data storage section 102 stores data (which is regularly rewritten by the CPU 107) for making the excitation control section 103 output given excitation light at a predetermined temperature. When the temperature sensor 101 detects ambient temperature, the data storage section 102 outputs data corresponding to the result of the detection. The excitation control section 103 outputs excitation light on the basis of the data it receives. The light receiving element 104 converts the excitation light into an electrical signal. The data storage section 105 stores photocurrent which flows at that time as data.
The excitation control section 103 first outputs excitation light on the basis of the data sent from the data storage section 102. The difference between the photocurrent which flows at that time and photocurrent stored in the data storage section 105 is provided to the excitation control section 103 via the differential amplifier 106. The excitation control section 103 adjusts the excitation light it outputs so as to eliminate this difference.
The conventional apparatus 100 performs not only temperature compensation but also degradation compensation by updating data with the CPU 107. However, this data update is not performed by detecting the actual state of the degradation of the excitation light source. Therefore, it will be difficult to update data by the CPU 107 at the time when the correlation between a drive signal and excitation light power begins to change. In addition, even if excitation light sources of the same type are used in a plurality of units, the state of degradation will differ among these units. Accordingly, it is impossible to accurately perform degradation compensation on the basis of data uniformly set for each unit. That is to say, it is virtually difficult for the conventional apparatus 100 to perform compensation for the degradation of the excitation light source.
Moreover, if a WDM optical signal is inputted and the number of different wavelengths contained in it changes, excitation light power corresponding to the number of wavelengths after the change must be outputted to curb the bad influence of a change in gain and a transient response. (For example, if the number of different wavelengths contained in the WDM optical signal reduces from forty to ten, then excitation light power must be changed quickly from excitation light power P40 corresponding to forty wavelengths to excitation light power P10 corresponding to ten wavelengths.) With the conventional apparatus 100, however, such a change of an input optical signal is not taken into consideration. Accordingly, the conventional apparatus 100 cannot handle an optical signal, such as a WDM signal, the level of which changes according to a change in the number of wavelengths contained therein. As a result, environments to which the conventional apparatus 100 is applicable are limited.